Intervals. A recent comprehensive review in two parts. Particular emphasis on VO2max, manipulation of work and rest periods to maximise time at VO2max. Are the intervals you're currently doing optimal? Programming considerations.

A great talk bringing a lot of stuff together - programming, intervals, a possible rationale for not spending too much time at threshold - with emphasis on mitochondrial adaptations: https://www.youtube.com/watch?v=NDY7YVpMrT0